Electrical heating element



my 2&5, T9381 D. MACKENZIE ELECTRICAL HEATING ELEMENT Filed Oct. l2,1935 INVENTOR z//vca/V/Vac/fwz/f' Jaya; WJ ATTORNEYS Patented July 2e,1938 t UNITED STATES PATENT @lorries 2 Claims.

This invention relates to electrical heating elements of the typeemployed in high temperature electrical resistance furnaces and itsobject is to provide a refractory heating element which is more efcientin operation and is capable of withstanding considerably highertemperatures over much longer periods of time than previous de vices ofthis type. p

Crdinary metallic heating elements compose of molybdenum,nickel-chromium alloys, etc., are used successfully in household andcommercial electrical appliances in which temperatures of less than9000o F. are encountered; but at higher temperatures, such as areemployed in electrical resistance furnaces, it is necessary to usenonmetallic heating elements composed of silicon carbide or variousother refractory compo: :ions which are readily obtainable on themarket. It is to this latter class of non-metallic heating elements thatthe present invention relates.

Non-metallic heating elements heretofore obtainable on the market areineffective at temperatures exceeding 2600c F., either failingcompletely around 2600 F. or being incapable of sustained operation atsuch high temperatures, This is due in large measure to theimpossibility of establishing and maintaining an effective contactbetween the heating element and its cooperating terminals, as allheating elements of this type contain countless small voids causingarcing and resulting pitting which impairs electrical conductivitity andsoon renders the element unt for further service. Another disadvantageof these heating elements as heretofore constructed is that the passageof sufficient current through the central heating zone always causesexcessive heating of the end portions of the element, which not onlyhastens the destruction of the terminals but also has a deleteriouseffect on the furnace itself.

. My invention provides an improved electrical heating elementcomprising` a rod or bar composed of non-metallic refractory materialhaving a portion composed of metal embedded in and coating the surfaceadjacent an end of the element for connecting same to anelectricalterminal. The metal coating is deposited in such a way, as by beingsprayed on the element under great pressure, that the metal completelyfills in the small voids in the underlying surface of the non-metallicrefractory material. After the metal coating is applied, it is smootheddown as by 'grinding to afnsh and fit suitable to the' seat'of thecooperating electrical-terminal. The

' smooth metal surface thus intimately combined (Cl. 20L-'76) with thenon-metallic refractory material provides a larger and more constantcontact between the heating element and lits terminal than has beenobtainable heretofore, thereby preventing loss of conductivity anddanger of arcing such as is experienced with all previous heatingelements of this general type. Heating elements employed in electricalresistance furnaces require terminal connections at both ends, and inthe case of such elements it is understood that I provide two embeddedmetal contacts as hereinafter described.

Another feature of my invention resides in the construction of arefractory non-metallic heating element or rod with a relatively narrowbody portion and an endor ends of enlarged crosssectional area, whichenlarged end or endsmay be coated with embedded metal contact surfacesas previously described. One advantage of this construction is that itpermits more current to pass through the enlarged ends of the rods tothe reduced central heating section thereof Without undue heating of themetal contacts and termina] connections, acting as a baille whichprotects the insulation as well as the exterior portions of the furnacefrom excessive heat. Furthermore, the enlargement at the ends of'the rodmakesit possible to keep the end portions much cooler than the centralheating section as it increases the available heat conducting surfacewhich may be cooled by suitable Water-jacketing of the furnace terminalconnections.

'My invention also provides an improved refractory heating element forelectrical furnaces and the like, composed essentially of a mixture offlint and coke. This heating element preferably comprises a mixture offlint and coke particles of substantially uniform size, molded to shapewith the aid of a binder, and heat treated to burn out the binder aswell as all other foreign matter. The resulting product is extremelyhard and resistant to fracture and is much more efficient and durable inuse than non-metallic heating elements of ordinary construction.

These and other features and advantages of the invention will beapparent from the following description taken in connection withchesp.,- cic embodiment shown in the accompanying drawing, wherein:

Fig. 1 is a fragmentary sectional view of the heating element, showingthe `usual method of mounting same in an electrical resistance furnace;and

Fig. 2 is a plan view of the heating element.

The heating element shown in the drawing comprises a non-metallicrefractory rod of circular cross-section having a relatively narrow bodyportion I and opposite end portions 2 of enlarged cross-sectional area.In the form shown, the extremities of end portions 2 are taperedoutwardly at 3, in the form of a truncated cone, and

these conical portions are coated at 4 with a layer of copper, nickel orany other suitable metal having satisfactory electrical conductivity.

The metal coating 4 is preferably sprayed on the tapered portions 3 ofthe rod in a molten state, as by means of a spray gun which bombarde theexposed surface of the rod with a ne stream of molten metal under greatpressure, completely lling in the tiny voids in the surface of thetapered portions of the rod and building up a coating of any desiredthickness, say from a sixtyfourth to an eighth of an inch thick. Thecoating thus formed is then ground smoothly to t the complementaryterminals 5 of the electrical furnace shown in Fig. 1. I prefer to applythe metal coating 4 to the flat ends of the rod as Well as to theconical peripheral portions thereof, and I have illustrated suchembodiment of the invention in Fig. 1, but the ends need not be treatedin this manner, nor is the invention limited to the conical contactsurfaces which are disclosed here for purposes of illustration only.

'I'he electrical resistance furnace shown diagrammatically in Fig. 1comprises a housing 6 of asbestos board containing suitable insulation'I such as Sil-O-Cel and a molded muflle 8 surrounding the heatingchamber 9. In the Vembodiment illustrated, the conical cup-liketerminals 5 at opposite sides of the heating chamber are integral withbrass tubes I0 which are mounted in porcelain sleeves II extendingthrough the walls of the furnace. The terminals are connected to asource of electrical current in any suitable manner. In operation, coldWater may be circulated through tubes I0 as by means of pipes I2 for thepurpose of cooling the terminal ends of the heating element. The smoothmetal surface 4 embedded in and intimately combined with thenon-metallic refractory material at each end of the heating elementcontacts snugly with its complementary terminal 5, preventing loss ofconductivity and' eliminating danger of arcing such as is alwaysexperienced when direct contact is made between the terminal and thenatural pitted surface of the heating element itself. Also, due to thespecial shape of the heating element, with a relatively narrow bodyportion I and end portions 2 of enlarged cross-sectional area, morecurrent will pass through end portions 2'to the central heating zonewithout excessive heating of the metal contacts and terminalconnections,

the enlarged end portions also being cooled more effectively than hasheretofore been possible. While the dimensions of the electrical heatingelement may vary widely as occasion requires, I prefer to make thecentral heating section I relatively long and the enlarged end portion 2relatively short in order to minimize the absorption of heat at the endsoutside the heating chamber of the furnace.

A molded refractory composition of matter which I have used with verysatisfactory results in the construction of the foregoing heatingelement is one composed of a specially treated mixture of intV and coke.Preferably, I take approximately 63 parts by weight of int to 26 partsbyweight of coke, both in a finely divided or powdered state, and mixsame thoroughly. .'hese proportions may vary anywhere from equal partsof flint and coke to 72 parts flint and 18 parts coke, but I ind thatthe best results are obtained with a mixture containing 63 parts flintand 26 parts coke, all of the stated proportions being by' Weight. 'Ihisthorough mixture is then heated, for example by packing same in a box ofsuicient size to hold the batch and embedding in the center of themixture a carbon or graphite hairpin resistor. The terminals of theresistor may then be connected to a standard 240 volt line and thecurrent turned on. This resistor can reach a temperature ofapproximately 4500? F., it being possible to build up such a temperaturesince the mixture under treatment has excellent heat insulatingproperties. The current is preferably left on until a temperature ofapproximately 3500 to 4500 F. is reached, after which the current isturned olf and the mix allowed to cool as by standing over night. Theresulting product is a mixture of greenish colored particles of powderedform which, when the above mentioned starting proportions of 63 partsiint and 26 parts coke are employed, analyzes: silicon 65.80%, ironoxide .87%, alumina 2.25%, magnesia .59%, and total carbon 29.90%. Isift this material to obtain granules of uniform size and then heat themixture, for example by placing same in a graphite crucible which Iplace in an electrical induction furnace, thoroughly sealing the top ofthe crucible before turning on the current. In a short time atemperature of approximately 4500 F. is reached, and I hold thistemperature until all the gases are burned out of the mixture, afterwhich the current is turned off and the batch allowed to cool overnight. The resulting material, which is in fine granular form containssilicon 61.76%, iron oxide .92%, alumina 5.10%, total carbon 31.85%, anda trace of magnesia. It will thus be seen that this second heattreatment not only burns out certain undesirable gases, but also effectsa markedchange in the chemical composition of the material.

The grains produced by the above treatment are mixed with a suitablebinder such as pitch or tar, in proportions suitable for molding, andheated to a temperature high enough to melt the binder and make aplastic mix. 'I'his plastic mixture is then placed in compression diesand molded to the shape of the resistance elements. After the moldedelements are removed from the die, I place them in a container and coverthem with carborundum to exclude air. The container is then placed in anelectric furnace and heated to a temperature of approximately 300 to 400F., which is done to burn out the binder. After the binder is burned outof the elements, I take a number of them, for example from one to adozen, and lay them across a carbon or graphite hairpin resistor, coverthem completely with a mixture composed of flint and coke in theproportions of the original mix described above, and bring thetemperature of the furnace to approximately 4500" F., holding thistemperature for about three hours,:after which the current is turned oliand the elements allowed to cool over night. In the morning the reds arecold and in such shape that they can be handled Without fear ofbreakage. The final product, free of gases,

binder and other foreign matter, contains silicon- 52.70%, iron oxide7.70%, alumina 4.20%, magnesiav .41% and total carbon 34.80%. 'I'heseproportions and all others given herein are by weight. The rods producedas described above are ground, for example in a universal grinder, to

the'desired shape or cone-fit on the ends, after 75 which they are readyfor the application of the metal coating l. v After this metal coatingis applied, preferably by spraying as described above, the metal endsare ground to fit the terminals i into which they make perfect contact.I find that, because of the composition of the heating elements, the useof uniformly sized particles of refractory material therein, and theelimination of the binder and other foreign matter therefrom, theseheating elements are far superior to any other non-metallic heatingelements heretofore produced.

From tests performed on various standard silicon carbide heatingelements. under conditions of scope of the appended claims.

The invention claimed is:

1. Method of making a refractory heating element for electrical furnacesand the like, which comprises mixing finely divided flint and coke inthe proportions of from equal parts of flint and coke to 'Z2 parts flintand 18 parts coke by weight, heating the mixture to a temperatureofapproximately 3500 to 4500 F., cooling the resulting mixture and sifting.same to obtain granules of substantially uniform size, heating suchuniform granules to expel gases, mixing the resulting gas-free granuleswith a plastic binder and molding to shape. and heating theplasticelement in the absence of air to burn out the binder.

2. Method of making a refractory heating element for electrical furnacesand the like, which comprises mixing finely divided int and coke insubstantially the proportions of 63 parts flint to 26 parts coke byweight. heating the mixture to a temperature of approximately 3500 to4500 F., cooling the resulting mixture and sifting same to obtaingranules of substantially uniform size, heating such uniform granules toa temperature of approximately 4500" F. until all gases are expelled,mixing the resulting gas-free granules with a plastic binder and moldingto shape, heating the plastic element in the absence of air to burn outthe binder. then covering said element I with a mixture of finelydivided flint and coke in substantially the proportions of the originalmix, and then heating to a temperature of approximately 4500 F. forabout three hours.

